Today I found a curious website, which I would like to share with you.
Strangely, I have long suspected that UFO's might be animals. But I was thinking of floaters, living in Earth's stratosphere. And recently I concluded that floaters are unlikely to exist on Earth, because there are only a few gases lighter than air, and these are either corrosive, or flammable, or unlikely to be biologically produced. (The most plausible gas would be NH3.)
The website suggests space-living animals, however. Which seems curiously plausible.